1 Scope

The inherent and installed flow characteristic, the relationship between flow coefficient and valve stroke, has been a subject of misunderstandings and endless debate with water transport and distribution systems operators.

The control valves must be analyzed from the perspective of their inherent characteristics and their system behavior (Installed characteristics). The “inherent flow characteristic” is the relationship between the flow rate through a valve and the travel of the closure member as the closure member is moved from the closed position to-rated travel with a constant pressure drop across the valve. The Inherent flow characteristics are determined under laboratory conditions. But, what interests hydraulic design engineers, control engineers, and automation engineers are the installed flow characteristics. “The Installed flow characteristics” include both the valve and pipeline effects. In other words, One crucial requirement in selecting the proper control valve is ensuring that the valve has the properly installed flow characteristic. The installed flow characteristic usually differs from the inherent due to changes in pressure drop: as flow increases, there is less pressure drop available across the valve. It’s generally desirable to have a linear installed characteristic.

2 Basic definitions

Basic terminology used herein is based on definitions stated in “Control Valve Terminology” [2] or applicable IEC standards.

3 Example A

It is a simple water transport system between two reservoirs, which are \(500 \; meters\) apart, and the elevation of the deposits is \(100\) and \(5 \; meters\), respectively. Before the entrance to the second tank, a control valve has been installed (see Flow Schematic).

Flow Schematic

Flow Schematic

Without the control system, the maximum flow possible the line gives and losses due to fittings and pipe are \(370.53\;l/s\). If a “Standard” plunger control valve with a flat portable cylinder is installed in the system, the maximum flow that will circulate through the transport systems will be \(369.18\; l/s\) -with a Zeta \((\zeta)\) of \(\zeta_{100\%}=3.8\) for the fully open valve and an additional gate valve with a Zeta of \(\zeta_{gate}=0.233\)

Kv and Zeta Value
depending on valve position
Valve Position Kv (l/s) Zv (-)
10% 1.4 257,537.8
15% 4.5 23,330.4
20% 10.6 4,307.8
25% 20.2 1,184.6
30% 33.8 422.4
35% 51.5 181.5
40% 73.2 89.9
45% 98.2 49.9
50% 125.8 30.4
55% 155.0 20.0
60% 184.9 14.1
65% 214.5 10.5
70% 243.2 8.1
75% 270.4 6.6
80% 295.8 5.5
85% 319.1 4.7
90% 340.4 4.2
95% 359.6 3.7
100% 376.8 3.4

Reference

[1]
American National Standards Institute ; International Society of Automation: ANSI/ISA-75.01.01-2007, Flow equations for sizing control valves.
[2]
American National Standards Institute ; ISA–The Instrumentation, Systems, and Automation Society ; Instrument Society of America: ANSI/ISA-75.05.01-2000 (R2005), Control Valve Terminology.
[3]
International Society of Automation ; American National Standards Institute: ANSI/ISA-75.02.01-2008, Control valve capacity test procedures.
[4]
International Society of Automation ; American National Standards Institute: ISA-75.11-2013 (2013), Inherent flow characteristic and rangeability of control valves.